Process for nickeliding,cobaltiding and ironiding base metal compositions



United States Patent PROCESS FOR NICKELIDING, COBALTIDING AND IRONIDING BASE METAL COMPOSITIONS Newell C. Cook, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Nov. 10, 1966, Ser. No. 593,270 Int. Cl. B23p 3/00 U.S. Cl. 29-194 11 Claims ABSTRACT OF THE DISCLOSURE Nickelide, cobaltide or ironide coatings are formed on specified metal compositions by forming an electric cell containing said metal composition as the cathode joined through an external electrical circuit to a nickel, iron or cobalt anode using a specified fused salt electrolyte maintained at a temperature of at least 700 C., but below the melting point of said metal composition in the substantial absence of oxygen. This cell generates electricity, but, if desired, and may be impressed on the circuit providing a cathode current density does not exceed 10 amperes/dmf This deposited metal diffuses into the base metal to form a tight adherent coating on the substrate composed of nickel, cobalt or iron and the substrate metal. This process is useful in the making of such coatings on the substrate metals.

This invention relates to a method for metalliding a base metal composition. More particularly, this invention is concerned with a process for nickeliding, cobaltiding or ironiding a 'base metal composition in a fused salt bath.

I have discovered that a uniform tough, adherent nickelide, cobaltide or ironide coating can be formed on a specific group of metals employing low current densities, that is, current densities in the range of 0.05-10 amps/dm.

In accordance with the process of this invention, the metal is employed as the anode and is immersed in a fused salt bath composed essentially of a member of the class consisting of the alkali metal fluorides, mixtures thereof and mixtures of the alkali metal fluorides with calcium fluoride, strontium fluoride or barium fluoride and containing from 0.015 mole percent of nickel fluoride, cobalt fluoride or iron fluoride. The cathode employed is the base metal upon which deposit is to be made. I have found that such a combination is an electric cell in which an electric current is generated when an electrical connection, which is external to the fused bath, is made between the base metal cathode and the nickel, cobalt or iron anode. Under such conditions, the iron, nickel or cobalt dissolves in the fused salt bath and nickel, cobalt and iron ions are discharged at the surface of the base metal cathode where they form a deposit of nickel, cobalt or iron which immediately diffuses into and reacts with the base metal to form a nickelide, cobaltide or ironide coating. In the specification and claims I use the terms nickelide, cobaltide, ironide and metallide to designate any solid solution or alloy of nickel, cobalt or iron and the base metal regardless of whether the base metal does or does not form an intermetallic compound with nickel, cobalt or iron in definite stoichiometric proportions which can be represented by a chemical formula.

The rate of dissolution and deposition of the nickel, cobalt or iron is self regulating in that the rate of deposition is equal to the rate of diffusion of the nickel, cobalt or iron into the base metal cathode. The deposition rate can be decreased by inserting some resistance in the circuit. A faster rate can be obtained by impressing a limited amount of voltage into the circuit to supply additional direct current.

The alkali metal fluorides which can be used in accordance with the process of this invention include the fluo rides 'of lithium, sodium, potassium, rubidium and cesium. However, it is preferred to employ an eutectic mixture of sodium fluoride and lithium fluoride because some free alkali metal is produced by a displacement reaction and potassium, rubidium and cesium are volatilized with the obvious disadvantages. It is particularly preferred to employ lithium fluoride as the fused salt bath in which the nickel, cobalt or iron fluoride is dissolved, because at the temperatures at which the cell is operated, lithium metal is not volatilized to any appreciable extent. Mixtures of the alkali metal fluorides with calcium fluoride, strontium fluoride or barium fluoride can also be employed as a fused salt in the process of this invention.

The chemical composition of the fused salt bath is critical if good metallide coatings are to be obtained. The starting salt should be as anhydrous and as free of all impurities as is possible or should be easily dried or purified by simply heating during the fusion step. The process must be carried out in the substantial absence of oxygen since oxygen interferes with the process. Thus, for example, the process can be carried out in an inert gas atmosphere or in a vacuum. By the term substantial absence of oxygen it is meant that neither atmospheric oxygen nor oxides of metals are present in the fused salt bath. The best results are obtained by starting with reagent grade salts and by carrying out the process under vacuum or an inert gas atmosphere, for example, in an atmosphere of nitrogen, argon, helium, neon, krypton or xenon.

I have sometimes found that even commercially available reagent grade salts must be purified further in order to operate satisfactorily in my process. This purification can be readily done by utilizing scrap metal articles as the cathodes and carrying out the initial metalliding runs with or without an additional applied voltage, thereby plating out and removing from the bath those impurities which interfere with the formation of high quality metallide coatings.

The base metals which can be metallided in accordance with the process of this invention included the metals having atomic numbers of from 27-29, 42-47 and 74-79 inclusive. These metals are, for example, cobalt, nickel, copper, molybdenum, technetium, ruthenium, rhodium, palladium, silver, tungsten, rhenium, osmium, iridium, platinum and gold. Alloys of these metals with each other or alloys containing these metals as the major constituent, that is, over 50 mole percent, alloyed with other metals as a minor constituent, that is, less than 50 mole percent, can also be metallided in accordance with my process, providing the melting point of the resulting alloy is not lower than the temperature at which the fused salt bath is being operated. It is preferred that the alloy contain at least 75 mole percent of the base metal and even more preferred, that the alloy contain mole percent of the base metal with correspondingly less of the alloying constituent.

I have also found it is advantageous to conduct the mealliding process in the absence of carbon. Tungsten and molybdenum, especially, form carbides which interfere with the diffusion of the iding agent, and the iding agents themeselves, especially iron, by their over-absorption of carbon, interfere with their own diffusion into other metals. I have found that carbon can be removed from the fused salt by operating it as a cell until the carbide coatings are no longer formed on the surfaces of the base metal.

Since nickel and cobalt can eb ironided and nickel can be cobaltided within the scope and teachings of this disclosure, nickel and cobalt and alloys of these metals are also included as materials which can be metallided by this process.

The form of the anode is not critical. For example, I can employ as the anode pure nickel, cobalt, or iron metal in the form of a rod or the iding agent can be employed in the form of chips in a porous copper or graphite basket. When a graphite basket is used to hold anode material, it is advantageous to shield the basket with a tightly woven metal cloth to prevent carbon fragments from getting to the cathodes.

In order to produce reasonably fast plating rate and to insure the fusion of the nickel, cobalt or iron into the base metal to form a nickelide, cobaltide or ironide, I have found it desirable to operate my process at a temperature in the range of from about 800 C. to 1200 C. It is preferred to operate at temperatures of from 900- 1100 C.

The temperature at which the process of this invention is conducted is dependent to some extent upon the particular fused salt bath employed. Thus, for example, when temperatures as low as 800 C. are desired, an eutectic of sodium and lithium fluoride can be employed. Inasmuch as the preferred operating range is from 900 C. to 1100 C., I prefer to employ lithium fluoride or a mixture of calcium and sodium fluorides as the fused salt.

When an electrical circuit is formed external to the salt bath by joining the nickel, cobalt or iron anode to the base metal cathode by means of a conductor, an electric current will flow through the circuit without any applied electromotive force. The anode acts by dissolving in the fused salt bath to produce electrons and ions. The electrons flow through the external circuit formed by the conductor and the metalliding ions migrate through the fused salt bath to the base metal cathode to be metallided, where the electrons discharge the metalliding ions and a nickelide, cobaltide or ironide coating is formed. The amount of current can be measured with an ammeter which enables one to readily calculate the amount of metal being deposited on the base metal cathode and being converted to the metallide layer. Knowing the area of the article. being plated, it is possible to calculate the thickness of the metallide coating formed, thereby permitting accurate control of the process to obtain any desired thickness of the metallide layer.

Although the process operates very satisfactorily without impressing any additional electromotive force on the electrical circuit, I have found it possible to apply a small voltage when it is desired to obtain constant current densities during the reaction, and to increase the deposition rate of the metal being deposited without exceeding the diffusion rate of the metalliding agent into the base metal cathode. The additional should not exceed 1.0 volt and preferably should fall between 0.1 and 0.5 volt.

Since the diffusion rate of nickel, cobalt and iron into the cathode article varies from one material to another, with temperature, and with the thickness of the coating being formed, there is always a variation in the upper limits of the current densities that may .be employed. Therefore, the deposition rate of the iding agent must always be adjusted so as not to exceed the diffusion rate of the iding agent into the substrate material if high efficiency and high quality diffusion coatings are to be obtained. The maximum current density for good nickeliding, cobaltiding or ironiding is amperes/dm. when operating within the preferred temperature ranges of this disclosure. Higher current densities can sometimes be used to form coatings with these iding agents, but in addition to the formation of a metallide coating, plating of the iding occurs over the diffusion layer.

Very low current densities (0.0l0.1 amperes/dm. are often employed when dilfusion rates are correspondingly low, and when very dilute surface solutions or very thin coatings are desired. Ofter the composition of the diffusion coating can be changed by varying the current density, producing under one condition a composition suitable for one application and under another condition a composition suitable for another application. Generally,

4 however, current densities to form good quality metallide coatings fall between 0.5 and 5 amperes/dm. for the preferred temperature ranges of this disclosure.

If an applied is used, the source, for example, a battery or other source of direct current, should be connected in series with the external circuit so that the negative terminal is connected to the external circuit, terminating at the base metal being metallided and the positive terminal is connected to the external circuit terminating at the metal anode. In this way, the voltages of both sources are algebraically additive.

As will be readily apparent to those skilled in the art, measuring instruments such as voltmeters, ammeters, resistances, timers, etc., may be included in the external circuit to aid in the control of the process.

Because the tough adherent corrosion resistant properties of the metallide coatings are uniform over the entire treated area, the coated metal compositions prepared by my .process have a wide variety of uses. They can be used to fabricate reaction vessels for chemical reactions, to make gears, bearings and other articles requiring hard,- wear-resistant surfaces. Other uses will be readily apparent to those skilled in the art as well as other modifications and variations of the present invention in light of the above teachings.

The following examples serve to further illustrate my invention. All parts are parts by weight unless otherwise expressly set forth.

Example 1 Twenty-three pounds of a mixture of 60 mole percent lithium fluoride and 40 mole percent sodium fluoride was placed in a monel liner (6" diameter x 17%" deep) fitted into a mild steel pot (6%" diameter x 18" deep). The pot was sealed with a cover plate of nickel plated steel (11" x 1") containing a water channel for cooling, two ports (2%." in diameter) for glass electrode towers, and two 1'' holes for a thermocouple probe and a gas bubbler. The steel pot was placed in an electric furnace for heating and the electrode towers attached and the mixed salt melted (M.P. 650 C.) under vacuum (less than 0.1 mm.). Argon was let into the cell and without adding any other salts to the cell, a nickel anode /z" x 6") and a copper cathode (6" x 1" x 0.020") was placed in the melted salt and the following run made at 750 C.

0 0.5 Current on. 0.5 Current off.

0 0 Sample out.

Upon lifting the copper strips from the salt, a white fog of sodium vapor evaporated from the surface. The copper strips had collected some black material which, on being washed off, left the strip bright and unchanged in color and with no weight gain or loss, indicating the sa t was very pure.

Thirty grams of nickel fluoride was then added to the fused salt bath and the following electrolysis made with a copper strip (6" x 1" x 0.020") at 750 C.

TABLE II Volts Current anode density, polarity amps/(1m.

-0. 030 0 +0.072 0.25 Current on. +0. 088 0. 25 +0098 0.25 Current ofl. +0. 012 0 0. 004 0 Sample out. w,

The sample had a silvery mat finish, was very clean and had gamed 0.112 gram compared to a theoretical 0.073 gram.

The bath temperature was raised to 800 C. and anproximately 0.5 mil thick and X-ray emission showed high other sample of copper strip of the same dimensions was concentrations of cobalt on the surface. nickelided in accordance with the data in the following Other metals were cobaltided and are summarized betable. low, Table V.

TABLE V Current Weight Percent Tirne, density, gain, coulombrc Metal Temp., C. amps/dm. grams efficiency Description of Coating Nickel 1 000 12 9 195 100 {0.2 1331 coat; bright, mat finish, slightly harder than nickel, all difiu- Platinum 1,000 20 0 75 043 100 {fln z h bright mat fi g y harder than p u all Gold 950 121 0. 90 0. 165 50 1 mil coat; dul l brown, grainy surface. M olyb d mum 1, 000 6 3 1 0' 060 36 {0.2algnliilucsn;a'tl;1 grrglgiltgirdignitgiimoderately hard, all diffusion, protective TABLE III Volts Current anode density, Example 4 polarity amps/rim.

Since iron is considerably more reactive than cobalt Time (min.):

1% 0 Cumnt and will displace cobalt from solution, the cobaltiding 62 .55% 0. 2 5 Current ofi. bath of Example 3 was converted to an ironiding bath by adding fifty grams of iron powder to the salt while This sample also had a silvery mat fini h a d had agitating the salt with an argon gas bubbler. After insertgained 0.193 gram of a theoretical 0.216 gram. The coating an iron anode 1 diameter X long) nd operating appfoximately 1/2 mil thick and i ing the cell for approximately 50 ampere. hours against exammauon appeared to be mostly dlfiuslon but scrap copper cathodes to further deplete the salt of cobalt with an outer layer of plating.

ions, the following run was made against a nickel strip p 2 (-6" x 1" x 0.020") to demonstrate the operating charac- A series of metal cathodes were employed using the teristics of the ironiding cell at 100.0" C. procedure of Example 1 and electrolyzed at 900 C. with the results shown in the following table.

TABLE IV Current Weight Percent Time, density, gain, coulombic mm. amps/dm. grams efficiency Description of Coating 60 1 0. 109 100 2 mil coating, bright, smooth, flexible, soft, all diffusion. 60 0. 2 0. 057 60 1 coating, bright, smooth, flexible, soft, all diffusion. 0 2 078 100 {gigignatmg bright mat finish, smooth, flexible, soft, all 4 Molybdennm 900 0. 6 0.172 70 g &s%f hard all 5 Tungsten 900 60 0. 5 1. 060 56 0.5 mil coating, dull mat finish, some diffusion, most plating Example 3 Volts Current The nickel fluoride dissolved in the sodiumlithium fluo- 45 523?; 39 7323 ride eutectic bath of Example 2 was displaced by adding lithium metal slowly to the salt at 900 C. Cobaltous o fluoride (60 gram) was then added to the salt and a cobalt 0. 5 Current on. anode (4%" x /2" x immersed 4" into the salt. '3 Current The bath temperature was raised to 960 and a 'strip of 8 copper 3" x x cobaltided as follows:

volts Current The sample, when lifted from the salt, was bright and anode density, smooth and had gained 0.175 gram of a theoretical gain p y p of 0.208 gram for the cathode reaction Fe+++2e- Fe.

Metalographic examination showed the coating to be 1 g C 0.5 mil thick and X-ray emission analysis showed it to be urrent on. L3 c rr nt ofi, predominately a mixture of iron and nickel but to also 0 contain some cobalt, indicating that longer clean-up was necessary to remove all the cobalt from the salt.

The sample came from the bath bright and smooth and Further electrolytic clean-up was performed on the salt had gained 50 mg, the theoretical amount for the cathode and then a series of other metals ironided as shown in reaction of Co+++2e- Co. The diffusion coating was ap- Table VI.

0 Sample out.

TABLE VL-IRONIDING Current Weight Percent Time, density, gain, coulombic Metal min. amps/elm. grams efliciency Description of coating Gold 3 3 0 0. 027 100 0.3 mil coat; shiny, smooth, soft, magnetic, all difluslon. Platinum.-- 7 4 0 0. 037 60 0.4 mil coat; shiny, smooth, very soft, magnetic, all diffusion. Copper 60 0. 4 0.020 20 0.3 mil coat; mat finish, smooth, harder than cooper, magnetic. Molybdenum 1,000 40 0. 8 0. 092 32 0.4 m11 coat; mat finish, smooth, very hard, non-magnetic, all difiusron.

Tungsten 1, 000 30 0 4 0.012 11 0.1 mil coat: mat finish, smooth, very hard ,slightly magnetic. Nickel 1, 000 500 0 1 0. 770 48 2.0 mil coat; bright, smooth, very soft, all diflusion.

Do 1, 100 10 1 5 0. 103 0.5 mil coat; bright, Smooth, very soft, all difiusion.

All the above samples were shown by X-ray emission analysis to be primarily coatings of iron and the substrate metal.

7 It will, of course, be apparent to those skilled in the art that modifications other than those set forth in the above examples can be employed in the process of this invention without departing from the scope thereof.

' What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method of forming a nickelide, cobaltide or ironide coating on a metal composition having a melting point of greater than 900 C., at least 50 mole percent of said metal composition being at least one of the metals selected with calcium fluoride, strontium fluoride or barium fluoride and from 0.01-5 mole percent of nickel fluoride, cobalt fluoride, or iron fluoride, said electrolyte being maintained at a temperature of at least 900 C., but below the melting point of said metal composition in the substantial absence of oxygen, (2) controlling the current flowing in said electric cell so that the current density of the cathode does not exceed amperes/dm. during the formation of the coating, and (3) interrupting the flow of electrical current after the desired thickness of the coating is formed on the metal object.

2. The process of claim 1, wherein the fused salt electrolyte consists essentially of lithium fluoride and the fluoride of the metal being deposited as the metallide coating.

3. The process of claim 1 which is also conducted in the substantial absence of carbon.

4. The process of claim 1 wherein the absence of oxygen is obtained by use of an inert gas in the cell.

5. The method of claim 1 wherein the anode metal is iron and the metal composition is cobalt.

6. The method of claim 1 wherein the anode metal is iron and the metal composition is nickel.

7. The method of claim 1 wherein the anode metal is cobalt and the metal composition is nickel.

8. The method of claim 1 wherein the metal composition is copper.

9. The method of claim 1 wherein the metal composition is molybdenum.

10. The method of claim 1 wherein the metal composition is tungsten.

11. A metal product produced in accordance with the process of claim 1.

References Cited UNITED STATES PATENTS 2,828,251 3/1958 Sibert et a1. 20439 3,024,175 3/1962 Cook 204-69 3,024,176 3/1962 Cook 20439 Re. 25,630 8/1964 Cook 204-39 3,232,853 2/1966 Cook 204-39 FOREIGN PATENTS 742,190 9/ 1966 Canada.

OTHER REFERENCES I. Electrochemical Society, vol. 112, No. 3, 1965, pp. 266272.

ROBERT K. MIHALEK, Primary Examiner R. L. ANDREWS, Assistant Examiner U.S. C1. X.R. 204-39 

